This invention relates to the addressing of liquid crystal displays (LCDs) of the kind in which ferroelectric liquid crystal material is provided in a thin layer between respective front and back supports. Usually these supports are transparent, to allow the display to be back lit, and each carries a respective array of transparent, linear conductors. Conveniently, though not necessarily, the conductors carried by the two supports comprise mutually orthogonal arrays, in row and column configuration, of individually energisable conductors.
Each intersection of a row and a column conductor defines an individual picture element (pixel) of the display, each of which pixels can be caused to assume one or the other of two different and stable conditions by the simultaneous application, to the relevant row and column conductors, of appropriate voltage waveforms.
In practice, it is usual to apply a conditioning (or so-called "strobe") waveform in turn to the row conductors carried by one of the supports and to apply data signals, indicative of the information to be displayed, in parallel and on a line-by-line basis, to the column conductors. Various expedients, including non-sequential addressing of rows and the duplication of column conductors to allow more than one row of data to be applied at once to the display, are used however to achieve practical displays capable of refreshment at rates sufficiently high to avoid flicker.
It is also usual to apply to each row conductor, at some time prior to the application of each strobe signal thereto, a blanking pulse which sets all pixels on the row into one of the two stable conditions. Thus, the data signal in each case has to provide, when combined with the strobe waveform, a combined waveform which either switches the pixel to its other stable state or leaves it in the state to which the blanking pulse set it. Thus the data signals are not so much `on` and `off` signals as `change` or `no change` indications.
It will be appreciated that the liquid crystal material affects light transmitted through or reflected from it in different ways depending upon the stable condition in question and thus that the overall display can be caused to affect, on a pixel-by-pixel basis, light transmitted through or reflected from it and that, because the pixels are conditioned in accordance with the information to be displayed, a two-dimensional display of the required information is achieved.
As is well known, polarised sheets are used to enable the distinction between the two states in optical terms to be seen, or at least to emphasise the contrast between those live states. It is also known that various expedients can be used to enable the display to exhibit colour and grey-scale.
The present invention is concerned primarily with the voltage waveforms used to address and condition the respective pixels and represents a significant departure from the practices that have been employed since the discovery of the ferroelectric effect in liquid crystal materials. It has as one objective to increase the operating speed of ferroelectric liquid crystal devices.
It has the further objective of reducing the voltages applied across the liquid crystal material, since there is merit in doing this both from the standpoint of ensuring that the switching activity of the liquid crystal material is not slugged or otherwise adversely affected by the application of voltages which are greater than necessary, and also because the cost of circuitry to generate the necessary waveforms for application across the liquid crystal material is substantially reduced.
A method of addressing the ferroelectric liquid crystal display which has been proved particularly beneficial is described in European patent No 306203. This method though not essential, is preferred for use with the present invention because the discrimination between switching and non-switching functions is particularly efficient.
A particular characteristic of this method is the fact that a voltage pulse for application to an individual pixel, which (as mentioned previously) is made up by the combination of voltages applied to respective elements of the two sets of conductors which sandwich the liquid crystal device, has to be of relatively low amplitude to cause switching and relatively high amplitude to leave a pixel unswitched. This is called the inverse mode of operation.
There are many patents, patent applications and other publications which describe addressing and/or switching waveforms for use in ferroelectric liquid crystal devices. Typical examples can be found in UK Patent Nos GB 2173336 and GB 2173629.
All of the currently published waveforms, however are characterised by exhibiting rectangular or square wave profiles. The present invention, as will be understood, departs significantly from this practise.
According to the invention there is provided a liquid crystal device comprising liquid crystal material capable of assuming a plurality of optically distinguishable states, applicator means for addressing individually resolveable regions of said material and for applying thereto electrical drive waveforms capable of causing the material at each of the various regions to remain in the state assumed thereby prior to the application thereto of a drive waveform or to assume another of said states, in dependence upon the nature of data to be represented by said device, and conveyed thereto in said electrical waveforms, the drive waveforms being of pulse-like form and of predetermined amplitudes and duration, wherein the drive waveforms also exhibit variations in pulse profile, which variations significantly influence the liquid crystal material to remain in one of said states or to assume another of said states.